City Unveils Long Awaited Stetson Report on Flooding in Mill Valley

On December 31, 2005, heavy rain penetrated ground already saturated by previous storms. Bolstered by rising tides, creeks overflowed their banks and cascaded across Mill Valley, causing intense flooding city officials say will likely occur only once every 50 years.

The incident sparked a lawsuit where victims across Marin sought more than $1 billion in damages, and Mill Valley residents demanded answers. Now, nearly eight years later, they have some.

Stetson Engineers, the firm the city hired to conduct a flood study of the Arroyo Corte Madera del Presidio watershed, presented its findings to the Mill Valley City Council at Monday’s meeting.

“Okay, I’ve got a report and it’s been eight years and people have been asking for this,” said Mayor Andy Berman. “And it’s going to cost us $8.2 million. It’s sad in a way.”

City Manager Jim McCann also acknowledged the public discontent over the timeframe.

“I think there certainly is concern, you might even describe it as outrage, that it has taken so long to figure out something so simple,” he said. “We’re built in a floodplain and it’s no surprise that we flood. Regularly.”

Part 1 and Part 2  of the Stetson report is available to the public on the city website. Based on surveys and hydraulic models using the peak flow of the 2005 storm, the study contains comprehensive details about flooding in the Arroyo Corte Madera del Presidio, particularly in the Sycamore Triangle, Miller Avenue and downtown areas, and suggests flood prevention measures for the next five to 100 years.

The City Council also heard an update on the Southern Marin Watershed Program, which is a collaborative effort by cities and the county to address 14 square miles draining into Richardson Bay. It includes the watersheds of Arroyo Corte Madera del Presidio, Coyote Creek, Ryan Creek, Sutton Manor Creek and East and West Creeks in Tiburon and will be used to create a comprehensive master plan for flooding in those areas.

The countywide watershed program is expected to issue a Watershed Master Plan by the end of 2015. That plan will cover myriad issues related to flooding and watershed issues, including identifying the impacts of sea level rise – the San Francisco Bay is projected to experience a sea level rise of about 16 inches by 2050 – and develop project concepts that could be adapted to rising tides.

The Stetson report will be factored into that master plan – scheduled to be completed by 2015 - and also used by Mill Valley to prepare for future flooding.

Part of the Stetson report includes a survey of nearly 1,100 structures in the floodplain, with information on which ones would be inundated up to the first floor in the event of a 10-year flood. That’s information that would have been great to have years ago, the councilmembers agreed.

“We do have that information, and I think we should provide it to homeowners, along with options to address the flooding potential,” said Councilmember Stephanie Moulton-Peters.

The five-year plans calls for replacements to the two Willow Street pedestrian bridges and widening the channel by about 150 feet. It also suggests removing the weir at the streamflow gauge, dredging the channel about 150 feet upstream of the gage, and building floodwalls or berms that range in height from .5 feet to 3 feet tall. The project is estimated $2.5 million.

The 10-year plan builds off of that work, and includes replacing the La Goma Street and Park Street bridges, raising the grade of Sycamore Avenue, and adding additional berms for an anticipated total of $8.2 million.

At 25 years the plan calls for a bypass channel around the watershed, followed by other measures at the 50 and 100 year marks. See the staff report for more information.

In July, the city plans to hold a public workshop on the Stetson report, and will discuss how the measures can be incorporated into the Watershed Program’s eventual master plan. Residents also have until July 31 to comment on the Stetson report before a final draft is presented to the City Council in early fall.

Potential funding options for projects stemming from either document include city funds, available county and district funds, and grants, and will likely take years to get off the ground.

“It’s going to take quite a bit more time before we get to a point where we’re able to move forward to some of the more exciting steps,” McCann said.